The use of fiber optic proximity probes or sensors utilizing bifurcated fiber optic bundles and sensors and detectors to determine the distance to a target is well known. Such devices utilize a light beam transmitted from a light source by way of a bundle of light conducting optical fibers (transmit fibers) from a light source to a target. The light is reflected off the target and returned along other optical fibers (receive fibers) in the bundle back to a light intensity sensor. The intensity of the returned light is a function of the distance between the proximity probe tip and the target, so the output of the light sensor can be a very precise measure of that distance, or more importantly changes in that distance. Examples of such systems can be found in U.S. Pat. Nos. 3,327,584 issued June 27, 1976; 3,940,608 issued Feb. 24, 1976; and 4,247,764 issued Jan. 27, 1981.
However, it has been found that due to variations in the operating parameters, particularly the reflectivity of the target, a manual adjustment was necessary i.e., increasing or decreasing the source intensity in order to maintain the accuracy of the distance reading once reflectivity had changed. While in certain applications, such manual readjustment was acceptable, in other situations where perhaps the probe is inaccessible or required to be fixedly mounted, such manual adjustment becomes impossible.
Accordingly, there arose a need to provide for automatically compensating for a change in reflectivity. While there have been many attempts to provide for such a feature, some of which have been found very satisfactory, such as that recited in U.S. Pat. No. 4,247,764, there still remains a need for a simple yet effective means of compensating for reflectivity changes having wide applications.
Since typical fiber optic sensors are operative only within a defined working distance to the target, to produce the maximum sensitivity and linearity possible, the working distance of the probe from the target is often 1-5 mils. Typical of the prior art is its inability to provide a measurement of gap size with high signal sensitivity down to the point of contact, independent of surface gap scale and input light intensity. In many applications such a limited distance may restrict the application of the device. This is true also in the opposite direction, typical prior art probes lose their accuracy if the distance becomes too great. While U.S. Pat. No. 3,940,608 provides for an optical extender to increase this working space, it does not provide for automatic compensation for a change in reflectivity. Accordingly, there exists a need for such a device which can simply, yet effectively provide these features in a single device.